1. Field of the Invention
The present invention relates to an electric powered vehicle, and more particularly to an electric powered vehicle equipped with a motor generator performing generation of a vehicle driving force and regenerative power generation at a time of regenerative braking.
2. Description of the Background Art
Hybrid vehicles have recently received attention as environmentally-friendly automobiles. A hybrid vehicle is an automobile that can generate a vehicle traveling power using an electric motor for driving the vehicle in addition to the conventional engine. In particular, in order to recover energy by regenerative power generation at a time of regenerative braking of the vehicle, a motor generator having both the functions of an electric motor and an electric generator is generally employed as an electric motor for the vehicle.
As for regenerative control of electric powered vehicles, Japanese Patent Laying-Open No. 2002-262411 (Patent Document 1) discloses a configuration of a speed control device for an electric powered vehicle such as an electrically operated golf car for avoiding the likelihood of skids. As the slope angle is larger in downhill traveling, the braking force of regenerative braking is increased, and the proportion applied to the drive wheels of the entire braking force including an electrical braking force and a mechanical braking force is increased, so that the drive wheel is locked, causing a skid. Specifically, the speed control device of the electric powered vehicle disclosed in Patent Document 1 is intended to improve traveling stability by limiting the wheel braking force by regenerative braking of the main motor when the gradient of a road is larger than a preset threshold value, thereby preventing a skid of the wheel due to locking.
Furthermore, International Publication WO97/10966 (Patent Document 2) also discloses a regenerative braking control device of an electric vehicle configured such that a regenerative braking force of an electric motor is controlled according to a gradient state, in order to avoid a wheel lock at a time of downhill travel and maintain vehicle traveling stability. In particular, Patent Document 2 discloses that a wheel lock is avoided by keeping the regenerative braking force on a down slope at the similar state as on a flat road, and thus traveling stability can be kept.
In addition, Japanese Patent Laying-Open No. 2000-32602 (Patent Document 3), Japanese Patent Laying-Open No. 2002-369578 (Patent Document 4), and Japanese Patent Laying-Open No. 2005-263061 (Patent Document 5) disclose a control configuration to limit regenerative power generation at a time of a temperature increase of an electric motor, in an electric powered vehicle such as an electric vehicle or a hybrid vehicle.
In electric powered vehicles, generally, regenerative power generation is performed by a motor generator in response to a brake operation by a driver to generate a regenerative braking force. However, not only power running of generating a vehicle driving force but also regenerative power generation at a time of regeneration causes the temperature of the motor generator (also referred to as a motor temperature hereinafter) to be increased by heat generated mainly in a coil winding. When the motor temperature rises, it becomes necessary to limit the current amount, that is, the output torque, so that the vehicle driving force that can be generated by the motor generator is limited.
Therefore, even with the regeneration limitation as disclosed in Patent Documents 1, 2, when a downhill with a relatively gentle gradient continues, the motor temperature is increased by the continuous regenerative power generation, and the output (power running) torque of the motor generator is limited at a time of uphill travel or flat-road travel after downhill travel, so that the motive power performance may not be fully exerted. This problem is expected, in particular, in a travel pattern in which downhill travel and uphill travel are alternately performed, as in a mountain road.
Furthermore, in the control configuration that limits the regenerative power generation at a time of temperature increase of an electric motor (motor generator) that generates a vehicle driving force, as disclosed in Patent Documents 3-5, an excessive increase of the motor temperature can be prevented. However, this configuration is not enough as regeneration control that can cope with aforementioned problem and can secure a vehicle driving force sufficient for flat-road travel or uphill travel after downhill travel.
If the aforementioned problem is addressed in view of specification design, the size of the motor generator is increased, for the increased thermal capacity and the enforced cooling structure are required to suppress a temperature increase of the motor generator. In addition, in order to secure traveling performance at a time of temperature increase, torque has to be secured by shifting to low gear for the entire vehicle, leading to a poor fuel efficiency at a time of high-speed travel.